Technical Field
The application of nuclear magnetic resonance (NMR) to the study and imaging of intact biological systems is relatively new. Like X-rays and ultrasound procedures, NMR is a non-invasive analytical technique which can be employed to examine lining tissues. Unlike X-rays, however, NMR is a non-ionizing, non-destructive process that can be employed continuously to a host. Further, NMR imaging is capable of providing anatomical information comparable to that supplied by X-ray CAT scans in any orientation without patient discomfort. On the other hand, the quality of projections or images reconstructed from currently known NMR techniques either rival or transcend those observed with ultrasound procedures. Thus NMR has the potential to be one of the most versatile and useful diagnosing tools ever used in biological and medical communities today.
NMR occurs when nuclei with magnetic moments are subjected to a magnetic field. If electromagnetic radiation in the radio-frequency region of the spectrum is subsequently applied, the magnetized nuclei emit a detectable signal having a frequency similar to the one applied.
Many nuclei have intrinsic magnetism resulting from its angular momentum, or spin, of such nuclei. Resembling a bar magnet, the spin property generates a magnetic dipole, or magnetic moment, around such nuclei. Thus, when two external fields are applied to an object the strong magnetic field causes the dipoles for such nuclei, e.g., nuclei with spin designated 1/1, to align either parallel or anti-parallel with said magnetic field. Of the two orientations, the parallel alignment requires the nuclei to store less energy and hence is the more stable or preferred orientation. The second applied field comprises radio-frequency waves of a precise frequency or quantum of electromagnetic radiation. These waves cause such nuclei to nutate or flip into a less stable orientation. In an attempt to re-establish the preferred parallel or stable orientation, the excited nuclei will emit electromagnetic radio waves at a frequency nominally proportional to the magnitude of the strong field, but specifically characteristic of their chemical environment.
NMR technology therefore detects radio-frequency signals emitted from nuclei as a result of a process undergone by the nuclei when exposed to at least two externally applied fields. If a third magnetic field in the form of a gradient is applied, nuclei with the same magnetogyric constant will nutate at different frequencies, i.e., Larmor frequencies, depending upon the location within the object. Thus, similar nuclei in an object can be detected discriminately for a particular region in said object according to their Larmor frequency corresponding to a particular magnetic field strength along the applied magnetic gradient, as demonstrated by the following equation EQU f.sub.o =.gamma.H.sub.o
wherein f.sub.o is the Larmor frequency, .gamma. is the magnetogyric constant, and H.sub.o is the applied magnetic field.
Several factors, however, limit the usefulness of NMR applications in vivo. In general, NMR is an insensitive radiologic modality requiring significant amounts of nuclei with magnetic moments to be present in an object. Consequently, not all nuclei in vivo are present in sufficient quantities to be detected by present NMR techniques. Further, not all nuclei found in vivo have magnetic moments. Some of the more common isotopes that do not have magnetic moments which are found in vivo include .sup.12 C, .sup.16 O and .sup.32 S.
Thus, current NMR applications in vivo are restricted to those nuclei that have magnetic moments and are sufficiently abundant to overcome the insensitivity of present NMR techniques. For the most part, in vivo NMR applications almost invariably concern themselves with imaging or detecting the water distribution within a region of interest derived from the detection of proton resonance. Other nuclei not only have lower intrinsic NMR sensitivities but are also less abundant in biological material. Consideration has been given, however, to the use of other nuclei such as .sup.31 P which represents the next best choice for NMR in vivo applications to its natural and abundant occurrence in biological fluids. For example, .sup.31 P NMR has been found to provide an indirect means for determining intracellular pH and Mg.sup.++ concentration simply by measuring the chemical shift of the inorganic phosphate resonance in vivo and determining from a standard titration curve the pH or Mg.sup.++ concentration to which the chemical shift corresponds. (Gadian, D. G., Nuclear Magnetic Resonance and its Applications to Living Systems, First Ed. Oxford Clarendon Press, pp. 23-42 (1982); Moon, R. B. and Richards, J. H., Determination of Intracellular pH by 31.sub.P Magnetic Resonance. J. Biological Chemistry 218(20;7276-7278 (Oct. 25, 1973)). In addition, 23.sub.Na has been used to image a heart perfused with a medium containing 145 mM sodium in vivo. Difficulties with these nuclei arise because of inherent sensitivity losses due to the lower resonant frequencies of these nuclei (Moon, R. B. and Richards, J. H., Determination of Intracellular pH by .sup.31 P Magnetic Resonance, J. Biol. Chem. 218(20):7276-7278 (Oct. 25, 1973).
Another stable element which is uniquely suited for NMR imaging is F because its intrinsic sensitivity practically commensurates with that of protons, it has a spin of 1/2 so as to give relatively uncomplicated, well resolved spectra, its natural isotopic abundance is 100 percent, it gives large chemical shifts, and its magnetogyric constant is similar to that of protons. Accordingly, the same equipment used for proton NMR can be used in vivo. However, F NMR applications are not used due to practical non-existence in biological materials of fluorine observable by NMR methods normally employed in studying biological systems. However, nuclear medicine procedures using a 18.sub.F positron emitter are well documented and include, for example, bone scanning, brain metabolism and infarct investigations using fluorodeoxyglucose, and myocardial blood flow and metabolism. Suggestions have been presented involving the study of vascular system disorders with F imaging (Holland, G. N. et al, .sup.19 F Magnetic Reson. Imaging, J. Magnetic Resonance 28:133-136 (1977)) and the localization/kinetics of fluorocarbon following liquid breathing. Further, in vitro canine studies investigating the feasibility of fluorine as an agent for NMR imaging of myocardial infarction have also been performed (Thomas, S. R. et al, Nuclear Magnetic Resonance Imaging Techniques Developed Modestly Within a University Medical Center Environment: What Can the Small System Contribute at this Point?, Magnetic Resonance Imaging 1(1):11-21 (1981)).
Studies directed to conformational equilibria and equilibration by NMR spectroscopy have been conducted, particularly with cyclohexane and fluorocyclohexane rings. In such applications, the position of the equilibria between conformational isomers and measurements of rates of equilibration of such isomers as a function of temperature have been determined. The studies, however, were dependent upon the implementation of known temperatures to determine the equilibria and equilibrium rates (Roberts, J. D., Studies of Conformational Equilibria and Equilibration by Nuclear Magnetic Resonance Spectroscopy, Chem. in Britain. 2:529-535 (1966); Homer, J. and Thomas, L. F.: Nuclear Magnetic Resonance Spectra of Cyclic Fluorocarbons. Trans. Faraday Soc. 59:2431-2443 (1963)). It has further been illustrated that 13.sub.C may be employed as a kinetic thermometer in a laboratory environment. This particular application requires the examination system to contain at least two chemically exchanging sites which correspond to one exchange process and an independent means of determining the kinetic parameters describing the exchange process in order for 13.sub.C to serve as a kinetic thermometer. Such application, however, is limited to determining temperature at coalescence and is, thus, operable at only one temperature for each independent exchange process as opposed to over a continuous range. The method is further employed as a calibration technique and its accuracy is inherently unreliable to be of practical significance (Sternhell, S. Kinetic .sup.13 C NMR Thermometer, Texas A&M U. NMR Newsletter. 285:21-23 (June 1982)). Unfortunately, NMR studies based on .sup.13 F or .sup.13 C require infusion in the body of molecules containing these atoms due to their very low abundance in vivo.
Temperature has been measured by means of the NMR spectrum of liquid samples for the purpose of calibrating the temperature control apparatus of an NMR spectrometer. Many features of the NMR spectrum, for instance chemical shifts, often show weak temperature dependence, and could be used to determine temperature (Bornais, Jr. and Browstein, S., A Low-Temperature Thermometer for .sup.1 H, .sup.19 F and .sup.13 C, J. Magnet. Reson. 29:207-211 (1978)). The peak separation and spin-spin coupling in the proton NMR spectrum of a liquid test sample changed by 1.75 Hz and 0.07 Hz, respectively, when the temperature was varied by 10.5.degree. C. In objects, such as animals, were the best obtainable spectral resolution could be 10 to 50 Hz or larger, and it is desired to measure temperatures to an accuracy of 1.degree. C. or 2.degree. C. or better, such as means of temperature measurement is inapplicable.
As to temperature in an animal, it is well known that abnormal fluctuations in temperature such as increases may reflect infection or hyperthermia, while decreases may represent ischemia or hypothermia. Thus, it is useful to measure temperature in an animal accurately, inexpensively and reliably. Furthermore, induced hyperthermia can also be used as an adjunctive cancer treatment.
In the past, temperature measurements have generally consisted of invasive and cumbersome techniques that often result in less than reliable measurements. Examples of such techniques comprise invading needles, electrical wires, cables, or instruments that must be inserted into a region of interest. Such penetrating procedures possess unfortunately the potential to cause chemical and biological contamination to the host. Thus, proper preparation and sterilization procedures are required to prevent transmittal and corrosive contamination when the instruments to detect temperature are reused. Another disadvantage inherent to the conventional techniques concerns the discomfort and inconvenience experienced from communication with penetrating probes. As to highly delicate structures, the temperature may be obtained but not without sacrifice to the integrity of the structure. Generally, the structure may be damaged, repositioned or its dimensions changed. Short circuiting of the employed instruments may add additional expenses and time to the procedure. The instrument itself when exposed to physical and chemical extremes may interfere with its reliability. Moreover, conventional techniques are unable to measure a continuous temperature field in an object or animal and, thus, the invasive and cumbersome procedure must be duplicated for each time or at each point in space a temperature measurement is desired, or employ simultaneously a large number of temperature sensors.
Non-invasive and non-destructive temperature imaging in biological systems may be useful in many disciplines. One important application is clinical hyperthermia (HT) which is being used as an adjunctive cancer treatment (Hahn, G. M., infra) Although very promising results have been obtained, the effectiveness and safety of deep-seated HT treatment has been limited, mainly due to a lack of temperature control (Gibbs, F. A., Hyperthermia Oncology, eds. Taylor and Frances, Phila, pp 2155-167 (1984)). Indeed, the effectiveness of a HT treatment depends upon the minimum temperature reached in the tumor (greater than 42.degree. C.) while safety considerations limit the maximum temperature that can be reached in normal healthy tissues (less than 42.degree. C.) (Hahn, G. M., Hyperthermia in Cancer, Planum Press (New York, 1982)). The temperature must be, therefore, monitored throughout the entire heated region with at least one cm spacial resolution and 1.degree. C. sensitivity (Hahn, G. M., supra).
A method to conduct non-invasive temperature monitoring by magnetic resonance imaging (MRI) was recently proposed which employs Tl temperature dependency (Parker D. L., Smith, V., Shelton, P., Med. Phys. 10:321 (1983); Dickinson, R. J., Hall, A. S., Hinde, A. J., Young, I. R., J. Comput. Assist. Tomogr. 10:468 (1986); U.S. Pat. No. 4,558,279 to Ackerman et al). MRI, a non-invasive and non-ionizing imaging method (Lauterbur, P. C. (1975) Nature 18,69-83) has the advantage of producing anatomical images of any part of the body in any orientation with high resolution. Contrast in MRI is defined by parameters mainly related to certain physical properties of water molecules. Temperature sensitivity of one of these parameters, namely, the spin-lattice relaxation time or Tl has been demonstrated in-vitro for different biological systems thereby suggesting the thermal imaging potentiality of MRI (Lewa, C. J., Majeska, Z., Bull. Cancer (Paris) 67:525-530 (1980) Parker, D. L., supra; Dickinson, R. J., supra). However, in general, precise Tl MRI measurements are difficult and the accuracy for temperature determination is limited. In most cases the accuracy is no greater than 2.degree. C./cm/5 min acquisition time. (Parker D. L., Smith V. and Shelton P., supra; Dickinson R. J., Hall A. S., Hinde A. J., and Young I. R., supra; U.S. Pat. No. 4,558,279).
Unfortunately, there are large variations in Tl between different tissues and for the same tissue between different subjects. This has been ascribed to the multifactorial nature of Tl (Bottomley, P. A., Foster, T. H., Argensinger, R. E., Pfeifer, L. M., Med. Phys II: 425-448 (1984). The applicability of this technique seems therefore to be limited because a relative change of at least 1% in Rl is needed to detect a 1.degree. C. change in temperature (Cetas, E. C., supra) and T.sub.1 measurements using MRI are difficult due to its sensitivity to environment (Young, I. R., Bryant, D. J. and Payne, J. A. Magn Res. Med. 2:355-389 (1985). U.S. Pat. Nos. 4,319,190, 4,558,279 and 4,361,807 also disclose methods of imaging chemical shifts in a body. However, these methods were not directly applied to the indirect measurement of temperatures in vivo. The use of chemical-shift resolved MRI has also been experimentally proposed but has severe limitations (Hall, L. D., Reson. 65:501-505 (1983)). Furthermore, all these techniques have failed for temperature monitoring in vivo, so that NMR was not considered as a likely temperature imaging method.
A variety of methods are available in the prior art for measuring the diffusion constant of the regents of a medium. One is that described by George et al "Translation on Molecular Self-Diffusion in Magnetic Resonance Imaging: Effects and Applications", in Biomedical Magnetic Resonance, published by Radiology Research and Education Foundation, San Francisco 1984. This method describes the measurement of the diffusion constant by comparing the relative effect of the diffusion of the studied medium and on a standard substance during different magnetic excitation sequences. This method relies on increasing the intensity of a section selection gradient which modifies the thickness of the studied section. Thus, this method can only be applied to objects which are finer than the finest section thickness obtained by sequences used and practically of not use in animal or human subjects. The sensitivity of this method to diffusion is also relatively limited.
Another method which lends itself to the measurement of temperatures in living tissues, including animals and humans utilizes relatively long echo times and effective gradients as a result of their intensity and position. In addition the exact determination of diffusion coefficients is obtained without a standard substance by basing the calculations on acquisition parameters. This method is described in a patent application entitled "Process for Imaging by Nuclear Magnetic Resonance" filed by Breton, E. A., LeBihan, D. and LeRoux, P. in France on June 27, 1985 (FR 85 09824) and in the U.S. on Dec. 24, 1986 under a Ser. No. 06/946,034, now U.S. Pat. No. 4,780,674, which is a Continuation-in-Part of U.S. application Ser. No. 823,522 filed Jan. 29, 1986, now abandoned the entire contents of which are incorporated herein by reference, with particular emphasis on the characteristics and steps of the method such as the utilization of basic sequences using Spin-Echoes to measure and image diffusion and longer and/or more powerful field gradient pulses to eliminate the effects of blood microcirculation.
An additional method is described in French patent application No. 86 13483 entitled "Method of Imaging by Nuclear Magnetic Resonance" by Breton, E. A., and LeBihan, D. on Sept. 26, 1986. This application was also later filed in the EPO on Sept. 21, 1987, in Japan on Sept. 25, 1987 and in the U.S. with a Ser. No. 07/100,261 on Sept. 23, 1987, now U.S. Pat. No. 4,809,701. These patent applications describe improvements in diffusion measurements and images which can be obtained when NMR excitation sequences and recording of NMR signals by synchronization with heart beats in living tissues. Diffusion measurements and images can be obtained quickly by using Steady-State Free Precession NMR. Yet another method is described in a patent application entitled "Precede and Imagerie des Movements Intravoxels par RMN dans in corps" filed in France by LeBihan, D. on Oct. 13, 1987 and has a Ser. No. 87 14098. This patent application contains an invention which is related to the publication LeBihan, D., "Intravoxel Incoherent Motion Imaging Using Steady-State Free Presession", Magnetic Resonance in Medicine 7:346 (1988). This is a method for the fast imaging of diffusion by using Steady State Free Precision NMR. The entire contents of the patent application and the above article are also incorporated herein by reference.
In view of the foregoing description of the limitations posed by prior art NMR temperature measuring techniques there is a clear need in the art, with particularly imminent application to cancer treatments for an improved method of determining in vivo the temperature coefficient and obtaining temperature images which is safe, non-invasive and can provide the sensitivity and reliability required of such measurements.